Impedance-matching tap-off coupler for wave transmission lines



NOV. 9, 1954 EDLEN ETAL 2,694,182

IMPEDANCE-MATCHING TAP-OFF COUPLER FOR WAVE TRANSMISSION LINES Ffiled Feb. 20, 1953 GEORGE 6. EDI-EN I HENRY U. DIAUDRA P 4 INVENTORS ATTORNEY United States Patent IMPEDANCE-MATCHING TAP-OF F COUPLER FOR WAVE TRANSMISSION LINES George G. Edlen, Silver Spring, Md., and Henry M. Diambra, Washington, D. C.

Application February 20, 1953, Serial No. 338,104

6 Claims. (Cl. 333-6) This invention relates to tap-ofi couplers for coaxial cable having a central inner conductor and a concentric braided outer conductor, the two being separated by suitable insulation.

It is often necessary in modern video, radio and television distribution systems to tap a coaxial cable transmission line at various points in order to transfer energy from the main coaxial line to branch lines. Since such connections must often be made in existing systems, it is obviously desirableto be able to tap into the main line without interrupting service thereon, as was necessary with some prior systems. It is also desirable to achieve such branch connection without the use of soldered joints, which are time consuming, require expensive, skilled workers, may cause an undesirable change in impedance due to the deposition of surplus solder, and may cause heat damage to the insulation. Since the work requires highly-paid skilled workers and is often done in inconvenient and exposed locations, it is desirable to produce a tap-off which can be applied as quickly as possible and with the minimum use of tools.

All of these objects are achieved according to our invention, which comprises conductive means encircling the customary outer insulating coating of a coaxial cable, so as to firmly grip the cable; at least one short prong on said conductive means arranged to pierce the outer insulating cover to make contact with the braided concentric conductor, an aperture in said conductive means internally threaded to receive a threaded piercing member comprising an outer conductive member and a sharp central piercing element insulated from said outer conductor but rigidly connected thereto by concentric insulating members, said central conductor comprising an integrallyincluded impedance element. When the piercing member is threaded into the threaded aperture, connection is automatically made to both conductors of the main coaxial cable. Suitable branch coaxial conductor connections are provided on the piercing member whereby, connection to the main cable having been established, a branch conductor can be readily attached to the tap-off. By the above means, we provide a solderless, quickly attachable tap-off requiring few or no tools, of high electrical efficiency, great mechanical strength and rigidity, and one which can be readily completely waterproofed.

The specific nature of the invention, as well as other objects and advantages thereof, will clearly appear from a description of a preferred embodiment as shown in the accompanying drawing in which:

Fig. 1 is a side view of the improved tap-off with a branch cable attached thereto, but not the main cable;

Fig. 2 is a plan view of the connection block of the tap-off;

Fig. 3 is a sectional view taken along line 33 of Fig. 2; and

Fig. 4 is an axial sectional view of a portion of the piercing member, showing a modified construction.

Referring to Fig. 1, a metal block 2 is provided with a semicircular groove 4 of a diameter corresponding to that of the cable which is to be connected, and extending axially as best shown in Fig. 3. The grooved face of block 2 is provided with a cover plate 6 suitably fastened thereto, for example, by means of a hinge 8 and screw 10, although obviously any other suitable fastening means may be employed. The cover plate 6 is also provided with a semicircular groove corresponding to groove 4 so that a conductor cable will be snugly held between the ICC block and cover plate when the two are fastened together about such cable.

The cable 15 with which this connector block is intended to be used is shown in cross-section in Fig. 3 and comprises a central stranded conductor 14 surrounded by insulating material 16 which is in turn surrounded by a conducting braid 18 forming the outer conductor of a coaxial cable, the braid being in turn covered by a suitable outer jacket 20 of insulating material. Branch conductor 12 is of similar construction, although usually of smaller diameter, and for the purpose of the present invention is fastened to piercing terminal 21 shown in section in Fig. 3. The piercing terminal 21 comprises an outer conductive shell 24 suitably externally shaped at its central portion to form a nut, as best shown in Fig. l. The lower portion of the shell in Fig. 3 is externally threaded to fit a correspondingly threaded aperture 5 in block 2 (Fig. 1) and is internally bored to receive a combined piercing member and impedance matching condenser made up of insulating cylindrical block 26 which is centrally apertured to receive conductor 28. Conduc tor 28 is perforated and slit at its upper or cable-connecting end somewhat after the fashion of a pin-vise to receive the central conductor 13 of cable 12 with a friction fit so as to establish a good connection therewith. At its right end, element 28 is somewhat enlarged in diameter to form one plate of a condenser, the corresponding opposite plate of which is formed by the suitably enlarged left end of piercing pin 30. A disk of insulating material 32 is cemented to one of the plate members prior to assembly and constitutes the dielectric of the condenser so formed. Alternatively, the disc 32 may be of resistance material, whereby the impedance element is a resistor. Pin 30 is suitably held in insulating cylinder 34, the whole assembly fitting snugly the internal bore of shell 22 and being retained therein in any suitable manner, for example, by a slightly spun-over edge 36 of the conducting material of shell 22 (Fig. 4). The pin 30 projects beyond the shell 22 a distance slightly more than one-half the diameter of the main cable 15 with which connection is to be established. Since the cable 15 has a conductive outer braid which must be pierced by the pin, a layer of insulating material 38 is provided on the outer stem of the pin except near the point. This insulating coating may be formed by dipping or painting the pin with suitable insulating varnish or enamel, or it may be formed as shown in Fig. 4 by forming a reduced portion of the piercing pin shank as shown at 40 and slipping over the point an insulating tube 41 of sufficient elasticity to be forced over the enlarged portion of the point and yet snugly fit the reduced portion. In large-scale mass production, insulating elements 34 and 41 could be molded as a unit around shank 40. Means are provided for connecting the branch cable 12 to the piercing terminal, said means comprising metallic nipple 42 internally bored to freely snugly receive the central conductor 13 and first concentric insulating layer 17 of cable 12, the central conductor being cut to such length as to establish good contact with element 28 when the cable is fully inserted as shown. The braid 43 and outer insulating jacket 44 are cut somewhat shorter than dielectric 41 and are forced over the outside of nipple 42 as shown. A deformable ring 46, or any other suitable clamping means is then clamped about the outer jacket 44 to establish good connection between the braid 43 and the nipple 42. It should be emphasized, however, that the step of connecting branch cable 12 to the piercing terminal 22 is ordinarily not performed until after the piercing terminal has been positioned in block 2 so as to establish proper contact with main cable 15.

In using the tap-off, block 2 will first be positioned at the desired location on the cable to which it is to be connected and firmly fastened thereto as by screwing down screw 10 (Fig. 1). It will be seen in Fig. 3 that the block is provided with two points 46 which project for a short distance into the channel or groove 4. These points 46 are of sutficient length to pierce the outer covering 20 of cable 15 and to establish contact with braid 18 of the cable but not sufficiently long to penetrate any appreciable distance into the dielectric 16 of the cable. In this way, not only is a good connection established between block 21 and the outer braided conductor 18 of the cable, but also, the position of the block is definitely and firmly established and either longitudinal or rotary sliding of the block upon the cable is definitely prevented. This is of importance because it insures that when piercing terminal 21 is threaded into aperture 5 of the block 2, point will be guided accurately toward the center conductor 14 of the cable on a diameter; otherwise, if rotary movement between the cable and block were permitted, the possibility would exist that the point might take a course through the insulation which would miss the center conductor 14. In practice, it has been found that in most instances, member 21 can be screwed into position with the fingers to establish contact.

It will be noted that the above described construction provides an impedance element, namely, the condenser or resistor which is located as close to the central conductor 14 as considerations of mechanical strength and rigidity permit. This is obviously desirable in tending to reduce insertion loss, and the provision of this advantage without any appreciable diminution in mechanical strength and rigidity of the piercing terminal is one of the advantageous features of the invention.

If it is desired to insure water-tightness in the assembly, all surfaces through which water might leak may be coated with suitable waterproofing compound prior to assembly. This would include the surface between coverplate 6 and block 2, the threaded connection of piercing connector 22 and the block, and the connection between nipple 42 of the piercing connector and branch cable 12. Another method of waterproofing the connection of the block to the main cable is to provide a layer of deformable waterproof plastic sheet material attached to the underside of the block, including channel 4, so that when the block is applied to the main cable and the cover plate 6 is firmly fastened by means of screw 10, the plastic material, which may be of soft rubber, is sufficiently deformed to insure a completely watertight connection.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of our invention as defined in the appended claims.

We claim:

1. Apparatus for coupling a branch coaxial line to an insulated coaxial transmission cable having a central inner conductor and a coaxial outer conductor spaced and insulated therefrom, comprising conductive clamping means having grooved means for firmly engaging a coaxial cable; and short piercing means for penetrating the outer insulating jacket of a cable held in said groove means, said clamping means having a threaded aperture axially intersecting said groove means, and a piercing terminal member having a threaded force-applying hollow shell member threadedly mounted in said aperture and a central conducting element mounted in said aperture, said element comprising a connector portion for engagement with the central conductor of a branch circuit coaxial cable and a piercing portion of sufficient length to engage the central conductor of a coaxial transmission line held in said groove, when said shell is threaded into said aperture, said two portions being aligned with their adjacent ends separated, and an impedance element in series with said two portions and included between and in firm mechanical force-transmitting engagement with said ends, said piercing portion being insulated from conductive contact with the coaxial outer conductor of an engaged transmission cable by a tough insulating layer surrounding and held by said piercing portion, and having a conductively exposed piercing point, a rigid, mechanically strong insulating member between said connector portion and said force-applying shell member in engagement with opposed surfaces of said connector portion and said shell member for transmitting a piercing force from said shell member to said piercing portion through said impedance element as said shell is threaded into said aperture.

2. The invention according to claim 1, and stop means on said piercing terminal member to limit the forward travel of said piercing member as said shell is threaded into said aperture to position the point of said piercing member for conductive engagement with the center conductor of an engaged cable.

3. Apparatus for coupling a branch coaxial line to an insulated coaxial transmission cable having a central inner conductor and a coaxial outer conductor spaced and insulated therefrom, comprising conductive clamping means having grooved means for firmly engaging a coaxial cable, and short piercing means for penetrating the outer insulating jacket of a cable held in said groove means, said clamping means having an aperture axially intersecting said grooved means, and a piercing terminal member having a threaded force applying outer hollow conductive shell having a portion threadedly engaged with said clamping means, said piercing member having also an inner conductive portion terminating at the forward end in an insulated pin having an uninsulated point of sufficient length to penetrate and conductively engage the central conductor of an engaged main cable when the threaded portion of said outer shell is in full threaded engagement with said clamping means, the inner conductor being rigidly held and supported by at least a portion of the outer shell, the rearwardly extending end of said inner conductor constituting the central conductor, and the rearwardly extending end of the hollow shell constituting the outer coaxial conductor of a coaxial terminal for attachment of a branch coaxial conductor, and a substantial circuit-balancing impedance element included between and in alignment with the ends of said inner conductor and within said outer conductor, the terminals of said impedance element being coextensive with said inner conductor, said insulation between said inner conductor and outer shell comprising a mechanically rigid insulating member in engagement with opposed surfaces of said conductors for transmitting a piercing force from said outer shell to said inner conductor as said outer shell is brought into full threaded engagement with said clamping means.

4. The invention according to claim 3, said opposed surfaces comprising a forward-facing surface on said outer conductor and a rearward-facing surface on said inner conductor, said insulating member extending between said forward and rearward surfaces so as to transmit mechanical force between them.

5. The invention according to claim 3, wherein said impedance element is a resistor, the terminals of the resistor being formed of the adjacent ends of aligned portions of the inner conductor.

6. The invention according to claim 3, wherein said inner conductor comprises two aligned portions of highly conductive material, the adjacent ends of said portions terminating in opposed spaced conductive areas, and a dielectric material between said areas, said terminating portions and dielectric material constituting a condenser element.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date.

1,067,024 Hall et al July 8', 1913 r 2,429,243 Snow Oct. 21, 1947 2,490,622 Cork Dec. 6, 1949 2,598,671 Boothby June 3, 1952 2,615,948 Kamen Oct. 28, 1952 2,640,878 Espley June 2, 1953 2,647,953 Rowe Aug. 4, 1953 2,677,108 Brady Apr. 27, 1954 FOREIGN PATENTS Number Country Date 455,196 Great Britain Oct. 15, 1936 OTHER REFERENCES Fletcher, R. 0., Rev. Scientific Instruments, vol. 20, December 1949, page 864. 

